1. Cross-Reference to Related Patent
This application relates to U.S. Pat. No. 5,707,684, titled xe2x80x9cMethod for Producing Micro-Optical Componentsxe2x80x9d issued Jan. 13, 1998 to Donald J. Hayes and W. Royall Cox, the patent being incorporated herein by reference in its entirety.
2. Field of the Invention
The present invention relates to a method of applying lenses to optical fibers which collimate transmitted light.
3. Background of the Prior Art
Optical fibers are increasingly used in datacom and telecom optoelectronic devices and systems for transmitting data or signals. Many applications require a xe2x80x9cconnectionxe2x80x9d between fibers or an array of fibers where there is a free space between them. An example of such use is where it is desired to transmit a signal from an optical fiber to a defraction grating or an optical switch. Defraction gratings, for example, are very delicate and cannot withstand contact or direct connections with optical fibers. Defraction gratings may be used in what might be called wave length division multiplexing which allows optical fibers to carry different signals and different streams of data at the same time. These actual uses are beyond the scope of this invention which merely relates to a new way of collimating the light exiting an optical fiber so that the light travels in straight lines from the end of a fiber or fibers to its destination. Prior art collimation of light exiting an optical fiber is typically achieved by mounting of a prefabricated collimating lens, such as an RGRIN (radial gradient index of refraction) rod at the end of the optical fiber at the appropriate distance from its tip or by thermally sculpting the tip of the fiber to achieve the desired collimating effect. These RGRIN rods require diffusion and they are very expensive to fabricate and install in a fixture in line with an optical fiber. This prior art thus has a disadvantage of both being difficult and costly to achieve. It would be desirable to be able to produce low-cost, high throughput fabrication of collimating microlenses on optical fibers, which would greatly facilitate design and assembly of optoelectronic systems and devices utilizing free-space coupling of fibers to optical components such as other fibers, detectors, gratings, prisms, etc. in both datacom and telecom systems.
This invention provides, for the first time, an inexpensive way of adding significant value to optical fibers, by utilizing the ink-jet printing method of dispensing optical material for automated, in-situ fabrication of collimating micro-optics at the ends of fibers. The flexibility of this data-driven method also enables variation of the printed microlens radius of curvature and the use of optical materials of differing properties (e.g., indexes of refraction), in order to achieve a range of collimating beam widths for differing types of fibers, e.g., single-mode or multi-mode, and of differing specifications with respect to core diameters, numerical apertures, etc.
The first step in fabricating collimating microlenses for the ends of optical fibers by means of ink-jet printing comprises selecting a desired microlens geometry. The specific geometry of a plano-convex microlens needed to collimate the output light from a given fiber is determined by ray trace modeling, using the fiber core diameter and core and cladding indexes of refraction, along with the desired beam collimation width, as input parameters. To achieve collimation of the output light, the lenslet must be located coaxially with the fiber core and offset from the fiber tip by a distance equal to the lenslet back focal length, so that its focal point is at the surface of the fiber core. The diameter of the microlens must be large enough to capture all of the diverging light from the fiber, and its radius curvature is determined by modeling to achieve collimation at the targeted beam width.
To achieve, at relatively low cost, the geometry required for fiber output collimation by the ink-jet printing method a hollow collet is provided which has an opening therethrough sized to accept an optical fiber wherein the collet has open lower and upper ends. The collets are cut from drawn tubes to several millimeters in length (e.g., 5 mm), preferably quartz tubes and fire-polished at both ends. The tip of an optical fiber is inserted into the open lower end of the collet leaving the tip of the optical fiber spaced from the upper end of the collet by a standoff distance which will place the focal length of the microlens to be formed at the tip of the optical fiber. The cleaved and sheathing-stripped end of the fiber is inserted into the collet until the distance between the fiber tip and the other end of the collet is at the required lenslet offset distance, using a microscope and mounting fixture. The optical fiber is fixed in the collet by means of a drop of UV-curing epoxy applied to the fiber at the lower end of the collet and cure bonded in place. After curing of the bonding adhesive, the fiber-collet assembly is mounted vertically, open end up, to the printing station substrate chuck and aligned to the print axis. The microlens material is preferably UV-curing optical epoxy. Droplets of liquid microlens material are deposited into the open upper end of the collet by means of an ink-jet printhead in drop-on-demand mode until the opening within the collet is filled and a microlens of the desired geometry is formed thereon. The microlens material is then cured by application of ultra-violet radiation and/or heat.
In a preferred embodiment of the invention, a ferrule is used to form microlenses individually and nearly simultaneously for an array of optical fibers. The ferrule has an upper-side and a lower-side, with an array of closely-spaced openings therethrough that are sized to receive an optical fiber. A single optical fiber can be passed through each opening so that an array of fibers can be placed in the ferrule to permit an array of lenses to be produced at nearly the same time. The tip of an optical fiber is inserted into the open lower end of a ferrule opening leaving the tip of the optical fiber spaced from the upper end of a ferrule opening by a standoff distance which will place the focal length of the microlens to be formed at the tip of the optical fiber. The cleaved and sheathing-stripped end of the fiber is inserted into the ferrule opening until the distance between the fiber tip and the other end of a ferrule opening is at the required lenslet offset distance, using a microscope and mounting fixture. The optical fiber is preferably fixed in the opening by means of a drop of UV-curing epoxy applied to the fiber at the lower end of the ferrule opening and cure bonded in place. After curing of the bonding adhesive, the fiber-ferrule assembly is mounted vertically, open end up, to the printing station substrate chuck and aligned to the print axis. The microlens material is preferably UV-curing optical epoxy. Droplets of liquid microlens material are deposited into the open upper end of the ferrule openings by means of an ink-jet printhead until the ferrule openings are filled and microlenses of the desired geometry are formed thereon. The microlens material is then cured by application of ultra-violet radiation and/or heat.